1. Field of the Invention
This invention relates to backup and recovery of data.
2. Related Art
Computer systems typically rely on mass storage systems to store and retrieve data generated or used by the computer system. File servers (“filers”) are one such set of computer systems that offer the ability to store and retrieve relatively large amounts of data, and to make the data highly available to clients and client devices that wish to access that data. Generally, filers can use magnetic, magneto-optical, or optical mass storage, so as to provide relatively rapid access to data. These types of storage are relatively fast and reliable, particularly when used with fault-tolerant techniques, such as a RAID (redundant array of independent disks) configuration.
Filers, while relatively reliable, are occasionally subject to corruption or loss of data. Known techniques for addressing this problem include maintaining copies of the data in a separate filesystem, one that hopefully will not lose data simultaneously with the original filesystem. These copies can be maintained either (a) in a second filesystem recorded similar to the first, that is, on magnetic mass storage; or (b) on a different type of mass storage medium, such as magnetic tape. The first of these known techniques is sometimes called “mirroring”; the second these known techniques is sometimes called “dump” (or “dump to tape”). The invention described in this application primarily relates to dump operations, but those of ordinary skill in the art will recognize, after perusal of this application, that the principles of the invention can be applied to other techniques for data backup, including mirroring and related techniques.
One problem in the known art is that the amount of storage recorded at filers has increased dramatically over time. It is presently not uncommon for storage in these devices to be measured in trillions of bytes (Terabytes), and for a dump operation to take many hours, and to use dozens of tapes for storage of an entire filesystem. If there is an error (either due to a problem at the filer, or due to a problem at the tape drive) in the middle of the dump operation, the entire dump operation is aborted and restarted at the beginning. For example, the dump operation sometimes fails before completion due to errors or interruptions, such as errors in writing to tape, power failures, filesystem errors at the filer, user interruption (accidental or otherwise), and the like. If the dump operation is aborted and restarted at the beginning, it is wasteful of the effort already expended during the dump operation that was aborted (and consequently more wasteful the more of the dump operation had been completed by then). Moreover, an error during the dump operation is more and more likely when the amount of information being dumped to tape is larger and larger and the dump operation is longer and longer. Thus, the average time it takes to complete a successful dump (after restarting due to errors, and finally running to completion) increases more than linearly with the size of the filesystem.
Accordingly, it would be advantageous to provide a method for the backup of data that is not subject to the limitations of the known art. This advantage is achieved in an embodiment of the invention that allows partial results of the dump to be preserved, and the dump to be restarted at a point relatively close to where the failure occurred.